Research Summary This study explores novel methods to assess how geographic and social contexts create unique profiles of risk for emerging adult HIV-negative men who have sex with men (MSM). HIV risk behaviors (sexual risk, substance use) are heavily influenced by geographic and social context of the neighborhoods that people live and frequent (e.g., crime; poverty; density of liquor stores, parks, churches, clinics). However, most studies that assess geographic influence, focus on a fixed location (e.g., home). However, young MSM do not stay in one place, and travel to different places to meet people and engage in risk behavior (e.g., bars, parties, sex clubs). There is a need to take a more dynamic and complex approach when trying to understand the geographic and social contexts of HIV risk. This proposal explores novel and innovative methods of assessing the influence of social places and their impact on risk behavior (e.g., sexual risk, substance use) among emerging adult MSM. To understand these mechanisms we will use a unique combination of geographic data captured through GPS monitoring and monitoring of partner-seeking app use (e.g., Grindr), and context-aware experience sampling using mobile technology that has participants answer brief questionnaires about their geographic and social environment triggered by GPS location or use of partner- seeking apps. In this application we propose to enroll 400 emerging adults MSM (ages 18-29) and track their GPS coordinates and use of partner-seeking apps for 2 months to create activity space maps of where they go and where they engage in risk behavior. The GPS coordinates from their activity space maps will be programmed in their cell phone and then we will conduct context-aware experience sampling assessments daily for 30 days (triggered by the GPS location of activity spaces or the use of a partner-seeking app). Context-aware experience questions will assess interpersonal interactions with social network members, location environment, drug use, alcohol use, and sexual risk. Results will provide tremendous insight into MSM's interpersonal interactions in social settings that will serve as the foundation for developing ecological momentary interventions (which can deliver tailored intervention components in real time contexts and situations) to reduce HIV risk and substance use of emerging adult MSM. Ecological momentary interventions developed using data from this study would allow us to provide precision health approaches through tailored interventions in real time when participants were at highest risk and in most need of support. This study provides us with unique data on interpersonal interactions in places that are socially meaningful that can help us improve the social and environmental contexts that put emerging adult MSM at risk.